Most modern color photographic printing papers employ silver halide emulsions having a high chloride content in order to obtain rapid processing rates relative to silver bromide emulsions. The composition of these silver halide emulsions is usually AgClBr, where the percentage of bromide is very low, typically around 0.5% to 5%, and usually about 1%. The presence of small proportions of bromide enhances photoefficiency of the silver chloride emulsions and enhances the adsorption of sensitizing dyes to the emulsion surface while still allowing for rapid processing. However, even with small amounts of bromide present, some sensitizing dyes do not adsorb well to these emulsions resulting in poor spectral sensitization. Also, some dyes are very sensitive to the bromide level. A high level of bromide sensitivity is undesirable because it can lead to variability during the color paper manufacturing process.
Color printing papers usually consist of at least three emulsions that are sensitized to blue, green and red light. Proper sensitization can be achieved by employing an appropriate sensitizing dye in each layer. Many common color photographic printing papers have a blue layer which is sensitized with a sensitizing dye so as to have a maximum sensitivity in the visible region at about 480 nm.
Color photographic printing paper is intended to generate a print from a photographic color negative. An important quality characteristic of color paper is color reproduction which is the ability to accurately portray the colors, or, more precisely, the hues of the original scene. Replacing a deep blue sensitizing dye which might sensitize at 480 nm or longer, with a dye that sensitizes at a shorter wavelength, can provide a color paper with improved color reproduction. For example, this can be attained by replacing a dye such as dye C-1 described below, which provides an emulsion with a maximum sensitivity in the visible region (".lambda.sens") at 480 nm, with a dye that sensitizes at 470 run or shorter.
However, using a shorter blue sensitizing dye typically results in a speed loss with normal printer exposures. One reason for this is that the energy output of the exposing device in many color paper printers diminishes at wavelengths shorter than 480 nm.
A cyano substituted sensitizing dye (comparative dye C-2 described below) has been disclosed in U.S. Pat. No. 4,942,121; 5,082,765 and 4,840,958. However, such a dye does not provides an emulsion with good sensitivity.
It would be desirable then, to provide sensitizing dyes that can provide a silver halide emulsion with a maximum visible light sensitivity at shorter than 480 nm, and preferably at 470 nm or shorter, which still provides the emulsion with good speed. Additionally, it would be desirable that the performance of such a dye does not vary too much with small changes in bromide levels in the silver halide emulsion.